# std::ranges::uninitialized_fill

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T > requires std::constructible_from<std::iter_value_t<I>, const T&> I uninitialized_fill( I first, S last, const T& value ); | (1) | (since C++20) (constexpr since C++26)
template< no-throw-forward-range R, class T > requires std::constructible_from<ranges::range_value_t<R>, const T&> ranges::borrowed_iterator_t<R> uninitialized_fill( R&& r, const T& value ); | (2) | (since C++20) (constexpr since C++26)

1) Copies value to an uninitialized memory area `[`first`,` last`)` as if by

for (; first != last; ++first)
`` ::new (`voidify`(*first)) std::remove_reference_t<std::iter_reference_t<I>>(value);
return first;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Equivalent to return ranges::uninitialized_fill(ranges::begin(r), ranges::end(r), value);.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | iterator-sentinel pair denoting the range of elements to initialize
r | - | the range of the elements to initialize
value | - | the value to construct the elements with

### Return value

As described above.

### Complexity

Linear in the size of the uninitialized memory area.

### Exceptions

Any exception thrown on construction of the elements in the destination range.

### Notes

An implementation may improve the efficiency of the `ranges::uninitialized_fill`, e.g. by using ranges::fill, if the value type of the output range is TrivialType.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms, (1,2)

### Possible implementation

    struct uninitialized_fill_fn
    {
        template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
            requires std::constructible_from<std::iter_value_t<I>, const T&>
        constexpr I operator()(I first, S last, const T& value) const
        {
            I rollback{first};
            try
            {
                for (; !(first == last); ++first)
                    ranges::construct_at(std::addressof(*first), value);
                return first;
            }
            catch (...)
            {
                // rollback: destroy constructed elements
                for (; rollback != first; ++rollback)
                    ranges::destroy_at(std::addressof(*rollback));
                throw;
            }
        }

        template<no-throw-forward-range R, class T>
            requires std::constructible_from<ranges::range_value_t<R>, const T&>
        constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, const T& value) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), value);
        }
    };

    inline constexpr uninitialized_fill_fn uninitialized_fill{};

### Example

Run this code

    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        constexpr int n{4};
        alignas(alignof(std::string)) char out[n * sizeof(std::string)];

        try
        {
            auto first{reinterpret_cast<std::string*>(out)};
            auto last{first + n};
            std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀");

            int count{1};
            for (auto it{first}; it != last; ++it)
                std::cout << count++ << ' ' << *it << '\n';

            std::ranges::destroy(first, last);
        }
        catch(...)
        {
            std::cout << "Exception!\n";
        }
    }

Output:

    1 ▄▀▄▀▄▀▄▀
    2 ▄▀▄▀▄▀▄▀
    3 ▄▀▄▀▄▀▄▀
    4 ▄▀▄▀▄▀▄▀

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed

### See also

ranges::uninitialized_fill_n (C++20) | copies an object to an uninitialized area of memory, defined by a start and a count (algorithm function object)
uninitialized_fill | copies an object to an uninitialized area of memory, defined by a range (function template)

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